The primary method for determining spherical refractive error of the human eye is to sequentially display targets to a patient and seek his/her responses to identify which object appears to be sharpest. A determination of the myopia (short-sightedness) or hypermetropia (long-sightedness) of the patient is made. The method is well established and has been used by optometrists for many years as the basis for determining the correct prescription for glasses and contact lenses.
The spherical refractive power (power of the lens required to correct ametropia) for the vast majority of people is between −5 dioptres and +2.5 dioptres. The accommodation range of a person (the range through which the refractive power of the eye can be varied) depends on his/her age and reduces with increasing age. Typically, the amplitude of accommodation of a 10 yr. old child is of the order of 10 to 14 dioptres, of a 20 yr. old around 9 to 12 dioptres, for a 40 yr. old it falls to between 4 and 6 dioptres, and for a 60 yr. old it falls to a value of less than or about a dioptre.
Notwithstanding the ubiquitous acceptance of the current method, a simultaneous presentation of targets with different vergences to the subject's eye would enormously simplify the measurement procedure for both the optometrists as well as the subject. It would also be valuable if a measure of the amplitude of accommodation is obtained at the same time. A simple and quick procedure to estimate the cylinder power and axis of the eye would be rewarding. It would be advantageous to have multiple vergence targets for each eye to conduct binocular refraction especially for spherical balancing. It is also highly desirable to have a truly three-dimensional target for testing stereopsis.
Various optical techniques for measuring characteristics of the human eye have been developed with advances in optical technology. One interesting approach is described in German patent application number 19514656 in the name of Occulus Optikgeraete GMBH. The described invention records a reflection hologram of the cornea of the eye which is then digitised for analysis.
Other approaches, such as that described in International publication number WO2002011612 in the name of Tracey Technologies LLC, use a light source to probe the eye. Reflections from the retina are analysed to determine aberration refraction.
Some techniques use multiple measurements and computer analysis to determine optimal comeal shaping for precise vision. Reference may be had to International publication WO199201417 in the name of Larry Horwitz for a description of this technology.
The prior art mentioned above is representative of the various optical techniques that have been applied to vision analysis and correction. They are universally complex and generally invasive. A much simpler and non-invasive approach is needed.